The present invention relates to image forming apparatuses such as copiers, printers, and facsimile machines and, more particularly, to a fixing rubber roller and a fixing device incorporated in such image forming apparatuses.
In a fixing device, normally, at least either a heating roller and a pressure roller is an elastic member or a rubber roller. The pressure roller is pressed against the heating roller with a predetermined pressure to form a nip. A transfer material having unfixed toner image is passed between the heating roller and the pressure roller to heat and fix the toner image. In general, the rubber roller is provided by heat-curing rubber on a circumferential surface of a rigid body or metal core roller to a predetermined thickness to mold the rubber and bond it to the core roller simultaneously.
However, since the rubber roller rotates while it is pressed against an opposite roller, both end faces of the rubber roller are bulgingly deformed by the pressure, and a problem arises, after use for a long time period, in that the end faces and the rubber layer comes off the circumferential surface of the core roller at the end faces and in the vicinity of the same.
In order to solve this problem, according to Japanese Patent Publication No. 1-20745B, both longitudinal ends of a rubber layer of a rubber roller is chamfered over a predetermined length. When the rubber roller is rotated while it is pressed against a roller provided opposite thereto, the chamfered portions are not in contact with the opposite roller or subjected to a contact pressure that is very small compared to a contact pressure applied to a region of the roller located inside the chamfered portions in the longitudinal direction of the roller. This reduces a pressure acting on end face portions of the rubber layer which can bulgingly deform the layer and prevents the problem that the rubber layer comes off the circumferential surface of the rigid core roller at the end face portions and in the vicinity of the same.
However, there is a problem in that the method involves a complicated processing step for forming the rubber layer on the circumferential surface of the rigid core roller using press cure, cutting the layer into a predetermined length, and processing the layer into a chamfered shape over a predetermined length from the longitudinal ends of the layer. In addition, there is another problem in that it is costly to stay with the processing of the rubber layer that is an elastic member because the process is difficult to perform and is not preferable in processing accuracy.
Japanese Patent Publication No. 42-14119B discloses a fixing device comprising a heating roller incorporating a heat source and a pressure roller which is pressed against the heating roller to form a fixing nip portion. The pressure roller is provided by forming a rubber layer on a circumferential surface of a hollow cylindrical rigid core roller. A driving shaft is axially inserted into the core roller, so that the pressure roller is rotatable around the driving shaft. The pressure roller is provided with a one-way clutch, so that a part of the pressure roller is allowed to undergo free rotation in a predetermined direction and to rotate at the same angular velocity as that of the driving shaft in the direction opposite to the predetermined direction.
The heating roller has a rubber layer formed on a circumferential surface of a hollow cylindrical rigid core roller and is pressed against the pressure roller to form the fixing nip portion. A rotational driving force is transmitted from one end of the driving shaft of the pressure roller. In this configuration, when the driving shaft is driven, the heating roller frictionally rotates the pressure roller at the nip portion to convey a sheet material bearing an unfixed toner image. During the conveyance, in order to prevent any change in the circumferential rotation speed of the pressure roller due to the change in the thickness of the sheet material or the shape condition of the nip portion, a configuration is employed in which the pressure roller is rotated at an angular velocity that is slightly higher than the angular rotation velocity of the driving shaft in the traveling direction of the sheet material.
Tear of a sheet material and distortion of an unfixed toner image are prevented when the heating roller frictionally rotates the pressure roller at the nip portion to convey the sheet material because the pressure roller is driven at substantially the same angular velocity as that of the driving shaft in the traveling direction of the sheet material by an action of the one-way clutch in case that a slip occurs due to unstable frictional rotation due to the width of the sheet material and the surface condition of the sheet material. It can be expected that a similar feature including a one-way clutch may be provided as part of a transmission unit such as a gear wheel that is provided separately rather than inside the pressure roller. However, such a configuration will result in a high cost because it makes the device complicated and large-sized.